On the HF bands the size of the vehicle is small enough in terms of wavelength that it really doesn't act as a ground plane but rather capacitive coupling to the earth. I don't think you'll notice a big difference in radiation pattern in either location. I have my whip mounted on the rear corner of the pick up bed in a stake pocket (thus there is no metal for 270 degrees around the antenna) and I haven't noticed any directivity. It is more important to keep the whip itself away from metal in order to minimize capacitive coupling to the vehicle. Low on the bumber is bad from the perspective that a good percentage of the whip runs close to the metal. Up on the edge of the bed and at the rear corner keeps it well away from surrounding metal.

Bob is giving you the correct scoop. While there is some directivity to side, back, front or side mounting, in the real world the differences are minimal. There are more important things to worry about. Like proper bonding, proper wiring, noise abatement, mounting the rig (or faceplate) is a safe and convenient matter, and permanently mounting any device to minimize dangers in the event of an accident.

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